1. Field of the Invention
The present invention relates to an optical system for correcting a light intensity distribution, and more particularly to an optical system for correcting the intensity distribution of divergent light to a uniform distribution.
2. Description of the Related Art
One example of an optical microscope is a confocal microscope. The confocal microscope can obtain slice images of a sample without making the sample into slice portions, and a correct three-dimensional image of the sample can be constructed from the slice images. Therefore, the confocal microscope is used in, for example, physiological reaction observation and morphology observation of living cells in the fields of biology and biotechnology, or surface observation of an LSI (large-scale intergration) in the field of semiconductors.
In such a confocal microscope, plural beam spots are produced from laser light, the sample is irradiated with the beam spots, and the sample is observed on the basis of fluorescence or reflected light from the sample. In this case, distribution uniformity of the intensity of the laser light (a Gaussian distribution is obtained with respect to a plane perpendicular to the optical axis) affects the intensities of the beam spots. In order to obtain only a uniform light flux in the vicinity of the optical axis of the laser light, therefore, an aperture plate having an aperture is disposed, and only a light flux which is passed through the aperture plate is used.
Also a confocal scanner has been disclosed in which a light intensity distribution correcting filter is placed between a collimating lens that converts divergent light emitted from a fiber end to parallel light, and an aperture plate (for example, see JP-A-2001-228402). The light intensity distribution correcting filter flattens the intensity distribution of light which is passed through the aperture of the aperture plate among incident light having a Gaussian light intensity distribution, and cuts off light other than the light passed through the aperture of the aperture plate.
In a configuration where the light intensity is corrected in this way, the quantity of available light is small, and therefore, in order to sufficiently irradiate a sample, a light source having a correspondingly large output power must be used. This causes stray light to be excessively increased. Therefore, the configuration is not suitable for a case where weak light is handled, such as fluorescence observation.
By contrast, in a configuration where a sample can be irradiated with light of a uniform intensity without reducing the light quantity, a light intensity uniformalizing lens is used (for example, see JP-A-11-95109).
FIG. 6 is a diagram of a confocal microscope disclosed in JP-A-11-95109.
Referring to FIG. 6, light emitted from a point light source 61 such as an optical fiber end is converted to parallel light by a collimating lens 62, the intensity distribution of the parallel light is uniformalized by a light intensity uniformalizing lens 63, and the uniformalized light is incident on a collecting disk 66 through an aperture 65 of an aperture plate 64. The point light source 61 is placed at the front focal point (focal length f) of the collimating lens 62.
A plurality of microlenses (for example, Fresnel lenses) 66a are formed in the collecting disk 66, and a plurality of pinholes 67a are spirally formed in multiple rows in a pinhole disk 67. The collecting disk 66 and the pinhole disk 67 are coupled to each other so that the pinholes 67a are located in the respective focal positions of the microlenses 66a. 
Laser light which is incident on the collecting disk 66 is collected by the microlenses 66a, and then passed through a beam splitter (not shown) to be collected to the pinholes 67a. The light which is passed through the pinholes 67a is collected by an objective lens 68, and then is irradiated on a sample surface 69.
The return light from the sample surface 69 is again passed through the objective lens 68 and the pinhole disk 67, and then reflected by the beam splitter (not shown) to enter a camera (not shown) via an imaging lens (not shown). An image of the sample surface 69 is formed on an image receiving surface of the camera.
In this configuration, the collecting disk 66 and the pinhole disk 67 are integrally rotated by a member 70, and the sample surface 69 is optically scanned (raster scanned) by the plural pinholes 67a, whereby a surface image of the sample surface 69 can be observed through the camera.
The light intensity uniformalizing lens 63 is a lens which maintains the quantity of the incident light entering from the collimating lens 62, and which uniformalizes the intensity of the incident light (for example, see JP-A-11-258544).
The light intensity uniformalizing lens 63 is placed between the collimating lens 62 and the aperture plate 64. The incident light entering the light intensity uniformalizing lens 63 has a Gaussian light intensity distribution, so that the intensity of the incident light is strongest in the vicinity of the optical axis, and the intensity is weaker as further separating from the optical axis. In the light intensity uniformalizing lens 63, a center portion where the incident light is dense is formed as a diffusing lens (concave lens) which diffuses parallel light, and a peripheral portion where the incident light is not dense is formed as a converging lens (convex lens) which converges parallel light. The light intensity uniformalizing lens 63 does not cut off light in a portion of the Gaussian distribution where the light intensity is low (the peripheral portion of the lens), and hence can maintain about 70 to 90% of the quantity of the incident light, thereby preventing a loss of the light quantity from occurring. The light emitted from the light intensity uniformalizing lens 63 is parallel light in which the light intensity distribution is substantially uniform.
In another laser light intensity distribution-converting optical system, by an afocal optical system of first and second groups (four-lens/two-group configuration) each configured by two lenses and having a positive refracting power, the light intensity distribution of an emission light flux which is parallel light is flattened, and the diameter of a flat distribution region is continuously changed by zooming (for example, see JP-A-3-75612).
In the configuration which is disclosed in JP-A-11-95109, and in which the light intensity distribution is uniformalized, however, the dedicated lens which converts the light intensity distribution, such as that disclosed in JP-A-11-258544 is used. Therefore, the configuration is produced with using a dedicated molding die, and a process of checking the curvature requires man-hours, resulting in that the configuration is expensive. Furthermore, a modulation for coping with the case of a different NA (numerical aperture) of a light source, and a change of the diameter of output light are hardly conducted. In the above, the NA is defined as NA=n sin θ where n is the refractive index, and θ is the divergence angle.
In the optical system disclosed in JP-A-3-75612, usual spherical lenses are used, but the four-lens configuration is necessary. This configuration adversely affects the cost and the space.